It is known to provide shaft furnaces for the reduction of zinc-containing ores or other metallurgical products containing zinc oxide, and to recover the zinc as zinc vapor which can be condensed from the gases emerging from the shaft furnace. The condenser can be supplied with molten lead at a temperature of about 500.degree. C. These systems are described, for example, in German Pat. Nos. 823,937; 824,259; 941,391; 943,726; and 965,954.
It has also been proposed to carry out the recovery of zinc by reduction and evaporation in a refractory-lined vertical cyclone. A slag chamber (British Pat. No. 971,729) is disposed under the vertical cyclone to collect the molten product which runs down along the cyclone wall. The gas conduit of the cyclone is connected to a condenser in which the zinc vapors are condensed.
One of the primary disadvantages of the use of the shaft furnace for the reduction of the oxidic zinc metallurgical solids is that the throughput of the latter is low per unit volume of the reactor. Furthermore, it has been found to be disadvantageous with respect to parameter control in that, for example, it is difficult to adjust and maintain a given gas atmosphere in the shaft furnace.
A shaft furnace requires that the solids bed have a uniform and high permeability for effective solid/gas contact and, to this end, the reducing agent is usually the coarse-grain coke which must be used in a relatively narrow particle size range. This requires pretreatment of the reducing agent and introduces another concern in the operation of a shaft furnace.
Finally, when zinc is volatilized in the shaft furnace, it is usually necessary to after-burn the exhaust gases to raise the exhaust gas temperature to a level which is sufficient to limit reoxidation of the metallic zinc. Reference may be had, in this connection, to the aforementioned German Pat. No. 941,391.
Prior-art systems for the recovery of zinc by a technique which involves a volatilization and condensation of the zinc all have the disadvantage that the latent energy content of the exhaust gas is not usually returned to the process. Moreover, the high temperatures of the exhaust gases of cyclone processes tend to result in difficulties in the operation of the condenser.